In recent years, a photographing camera unit has been available wherein an ultrasonic wave is transmitted to an object to be photographed for distance measurement and on the basis of a result of the distance measurement, a taking lens is rotated for its axial movement. When using the ultrasonic wave, however, an echo level changes drastically even for an object being still at a fixed distance under the influence of such factors as flow and temperature of atmospheric air, and a distance measuring signal is difficult to detect, resulting in erroneous distance measurement dependent on atmospheric air conditions. In addition, errors in distance detection due to noises tend to occur and acurate focal point adjustment will be difficult.
Where the distance to a target object varies, on the other hand, the echo level returned from the object varies with the distance in such a manner that when the object is at a close distance, the echo level becomes large and when at a remote distance, the echo level becomes small, thus giving rise to inconvenience that the distance detection is sometimes impossible for an object being at a remote distance.
For these reasons, highly accurate distance measurement can not be expected through distance measurement by one time. Further, in the event that, even with an object being distance within a focal point adjustable range, an echo signal can not be detected under the influence of an atmospheric air condition, an erroneous operation results wherein the taking lens is rotated for its axial movement corresponding to an infinite distance.